The invention relates to microsphere particles.
Hollow microsphere particles have a wide variety industrial and biomedical uses. However, the formation of uniform and regular shell structures, as well as control over the shell thickness, is difficult to achieve using present methods, thereby restricting the uses of such particles.
The invention features hollow microspheres and core-shell microsphere compositions with consistent shell thickness using methods, which allow controlled formation of a polymeric shell. The thickness of the polymeric shell preferably varies less than 10%, more preferably less than 5%, more preferably, less than 1%, and most preferably less than 0.5%. The variability in the thickness of the polymeric shell is determined by measuring the thickness at two or more points on the microsphere and calculating % divergence.
Shell thickness is controlled by the length of polymerization and is varied to provide microspheres for divergent applications such as drug delivery or synthetic pigment preparation. Duration of the polymerization step is directly proportionate to the length of the polymer chains, and thus, shell thickness. Shell thickness is in the range of 100-1000 nm. In preferred embodiments, the shell thickness is in the range of 150-250 nm. Alternatively, the shell thickness is in the range of 350-450 nm or in the range of 550-650 nm. Preferably, the microsphere is substantially devoid of silica. For example, the microsphere contains less than 10%, more preferably less than 5%, more preferably, less than 1% silica by weight.
The microspheres contain pores. A pore is a void in the polymeric shell through which a composition may gain access to the hollow portion of the microsphere. The microspheres have a certain porosity, and the porosity is varied depending on the size and composition of the substrate used to make the sphere. Pore size is varied depending on the size and nature of the composition to be loaded into the hollow center of the sphere as well as by changing the amount of crosslinking agent added during polymerization. For example, the addition of increasing amounts of a crosslinking agent produces microspheres with decreasing pore size. Pore size is also affected by the addition of a foaming agent, i.e., addition of a foaming agent during production of the shell increases pore size. For example, a pore has a diameter in the range of 10-500 nm.
Microspheres are useful as synthetic pigments, drug delivery vehicles, and protecting agents. For example, the microspheres are used in place of titanium dioxide, i.e., as a synthetic pigment, because an empty microsphere in solution appears white. Organic dyes are encapsulated in a hollow microsphere to produce a synthetic pigment of a desired color. Empty or dye-encapsulated microspheres have several advantages over standard titanium dioxide-based paints or dyes, e.g., improved color clarity or trueness.
The microsphere is also useful as protecting agent. For example, a light-sensitive compound (e.g., a photo-bleachable dye) is loaded into a hollow microsphere to protect its degradation from exposure to light or chemicals prior to use. The compound is released from protection by disrupting the microsphere, e.g., by crushing the sphere or contacting the sphere with a solvent.
In addition to industrial applications, microspheres are used as delivery vehicles for therapeutic agents such as polypeptides, antibodies, enzymes, small molecule drugs, or nucleic acids.
The nature of the polymeric shell is varied to accommodate various uses of the hollow microspheres. The microsphere shell typically contains styrene, methacrylate, or any polymer with a high glass-transition temperature (Tg). The shell contains a polymer resulting from the polymerization of one or more monomers selected from the group consisting of acrylonitrile, styrene, benzyl methacrylate, phenyl methacrylate, ethyl methacrylate, divinyl benzene, 2-hydroxyethyl methacrylate, cyclohexyl methacrylate, p-methyl styrene, acrylamide, methacrylamide, methacrylonitrile, hydroxypropyl methacrylate, methoxy styrene, N-acrylylglycinamide, and N-methacrylylglycinamide. Alternatively, the shell contains a co-polymer (random or block) selected from the group consisting of styrene-PMMA, benzyl methacrylate-PMMA, styrene-PHEMA, styrene-PEMA, styrene-methacrylate, and styrene-butylacrylate. The strength and durability of the polymeric shell is increased by crosslinking polymer chains.
The invention also includes methods of making hollow microspheres by providing a substrate containing a plurality of hydroxyl groups and attaching an initiator agent to the hydroxyl groups to form attached initiator agents. Any solid substrate, which is characterized as containing hydroxyl groups on its surface and is dissolvable (following polymerization of the shell) is suitable. For example, the substrate is silica, alumina, mica, or a clay composition. Alternatively, the substrate is a crystal, which has been coated with a silica. The initiator agents react with a polymerizable unit under polymerization conditions to form a polymer shell over the substrate. The polymerization is confined to a surface of the substrate. A polymer chain is initiated at the initiator agent and is extended away from the substrate during polymerization. To remove the substrate from the polymeric shell (to yield a hollow microsphere), the substrate is contacted with an etching agent for a time sufficient to allow for elimination of the substrate from the polymeric shell. An etching agent is a composition which removes a solid substrate from the center of a polymer-coated substrate, leaving a polymeric shell. Preferably, at least 85% of the substrate, more preferably 95%, more preferably 99%, and most preferably 100% of the substrate is removed from the core of the sphere. Etching agents include bases or acids, e.g., hydrochloric acid (HCl), hydrogen fluoride (HF), sulfuric acid (H2SO4), sodium hydroxide (NaOH), potassium hydroxide (KOH). Alternatively, the substrate is metal, and the etching agent is an oxidizing or reducing agent. For example, a silica substrate or mica is removed by etching with HF, and an alumina or clay substrate is removed by etching with KOH. Optionally, the method includes a step of exposing the polymer shell to a crosslinking agent.
The polymerizable unit is a monomer selected from the group consisting of acrylonitrile, styrene, benzyl methacrylate, phenyl methacrylate, ethyl methacrylate, divinyl benzene, 2-hydroxyethyl methacrylate, cyclohexyl methacrylate, p-methyl styrene, acrylamide, methacrylamide, methacrylonitrile, hydroxypropyl methacrylate, methoxy styrene, N-acrylylglycinamide, and N-methacrylylglycinamide or a co-polymer selected from the group consisting of styrene-PMMA, benzyl methacrylate-PMMA, styrene-PHEMA, styrene-PEMA, styrene-methacrylate, and styrene-butylacrylate. Thickness of the developing polymeric shell is controlled by the length of polymerization.
The invention also includes a core-shell composition. A core-shell composition is a composition, which contains at least two structural domains. For example, the core domain is encased in the shell domain, and the shell domain is characterized as having different physical and chemical properties than the core. The core portion contains a first compound, and the shell contains a second compound (which is not present in the core portion). The core and shell differ by the presence or absence of at least one compound. A method for preparing a core-shell composite includes the following steps: providing a microsphere substrate; contacting the microsphere substrate with a polymer nanosphere to yield a colloidal assembly; and heating the assembly to yield a core-shell composite.
An alternative method for preparing a hollow microsphere includes the following steps: providing a microsphere substrate; contacting the microsphere substrate with a polymer nanosphere to yield a colloidal assembly; heating the assembly to yield a core-shell composite; and exposing the composite to an etching agent for a time sufficient to allow for removal of a core composition, e.g., silica, from the shell polymer composition to form a hollow microsphere.
A colloidal assemby is an organized structure of two or more particle types. For example, the assembly is organized such that the nanospheres are assembled onto the surface of a microsphere. Preferably, the microsphere is 1-100 xcexcm in diameter; more preferably, the microsphere is less than 75 xcexcm in diameter; more preferably, the microsphere is less than 50 xcexcm in diameter; and even more preferably the microsphere is less than 25 xcexcm in diameter. For example, the microsphere is 3-10 xcexcm in diameter. The nanosphere is 1-1000 nm in diameter. Preferably, the nanosphere is less than 500 nm; more preferably, the nanosphere is less than 250 nm. For example, the nanosphere is 100-200 nm in diameter.
The nanospheres and/or microspheres are optionally modified to contain a reactive substituent. Preferably, the microsphere and nanosphere contain different substituents, which associate, bind, or react with one another. For example, the nanosphere contains an amine-modified polymer, e.g., an amine-modified polystyrene (PS), and the microsphere comprises an aldehyde-modified composition, e.g., glutaraldehyde-activated silica. The microsphere substrate contains silica, alumina, mica, or clay. In another example, the nanosphere contains avidin and the microsphere contains biotin, or the nanosphere contains biotin and the microsphere contains avidin. The nanosphere may contain one type of polymer or a mixture of polymers. For example, the nanosphere contains PS, PMMA, or both. The microspheres are optionally contacted with a mixture of different nanospheres, e.g., a mixture of PS nanospheres and PMMA nanospheres, to yield a composite polymer shell. The ratio of different polymer nanospheres is varied to achieve a desired effect, e.g., strength or porosity. For example, the ratio of PS:PMMA is 50:50, 100:1, 10:1, 5:1, or 2:1.
The colloidal assembly is heated to a temperature greater than the Tg of the polymer nanosphere to melt the polymer nanospheres. The polymer flows over the microsphere surface resulting in an essentially uniform coating, i.e., the thickness of the polymer shell varies less than 10% over its entire surface. For example, the colloidal assembly is heated to at least 100xc2x0 C. To melt PS and/or PMMA nanospheres, the colloidal assembly is heated to 170-180xc2x0 C.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.